1. Field of Invention
This invention relates to the field of adhesive bonding and laminating. Specifically, the present invention relates to systems and methods for bonding and laminating using atmospheric pressure, commonly known as vacuum bagging.
2. Description of the Related Art
Vacuum bagging is one of a variety of methods used for the bonding and laminating of manufactured parts. Generally speaking, the part being bonded or laminated is surrounded by an airtight membrane or diaphragm; a vacuum is pulled on that membrane; and the surrounding pressure forces the membrane tightly to the part. Once the part is vacuum bagged it is allowed to cure for a specific period of time depending on its composition. Curing can take place in an oven, autoclave, at room temperature or in a mechanical press.
In adhesive bonding two parts are assembled with a layer of uncured adhesive between them. Typically, a film adhesive is used. The process then applies atmospheric pressure (up to 14.7 psi, depending on altitude). The pressure forces the adhesive layer and the parts into intimate contact thus ensuring good bonds. The vacuum draws out some volatiles and trapped air, resulting in a low void content. Both help to improve resin flow. Vacuum bagging produces strong parts with little air entrapment. The process is adaptable to complex, three-dimensional shapes thus eliminating the need for expensive matched-die molds and presses.
All vacuum bags are constructed in essentially the same way. Before starting a release agent is applied to the tool. This may be a liquid release coating, a wax, or even a solid barrier such as Teflon® tape. When applying the release, the edges of the tool must be masked so the bag sealant tape (BST) will stick to the tool.
BST is a putty-like material, which comes in rolls, usually ½ inch wide, with a release paper on one side. The tape is pressed against the tool, leaving the release paper on until it is time apply the bag. The tape usually goes on after the assembly to be bonded is placed on the tool.
Once the assembly is in place the bag is applied. The first item to go down is a peel ply, which is optional. Peel plies are a tightly woven fabric, often nylon, usually impregnated with some type of release agent. The peel ply may stick to the assembly, but it will pull away without too much difficulty. For adhesive bonding, the peel ply is typically a solid sheet.
After the peel ply comes a layer of release film. This is a thin plastic, which has been treated so it will not bond to the laminate. It is highly stretchable so it can conform to complex geometries.
At least one layer of bleeder cloth goes above the release film. Bleeder is a thick, felt-like cloth. It provides a continuous air path for pulling the vacuum. If the bag wrinkles against the hard assembly, it will trap air. The breather prevents this from happening.
The bag is the last item to be placed. It is a relatively thick plastic layer, available in different amounts of conformability. The bag is usually applied along one edge at a time. One starts at one corner and presses the bag into the BST, removing the release paper from the tape as one moves along the edge. It is important to be careful not to get any wrinkles in the bag or it will leak. Pleats are required for anything but flat or simply curved structures. The bag may be heat sealed instead of using the BST.
Finally, a vacuum port is installed through the bag. The base of the port goes inside the bag. A small cross is cut in the bag for the attachment flange to fit through. If the tool has an area for the port, there must be a breather path from the port to the part. If the port goes on the part itself, several layers of breather are placed under the port to prevent print-through.
While vacuum bagging is a good and inexpensive method of fabricating adhesively bonded assemblies, because of the relatively low pressures employed, a fairly thick bond line results and air can still be trapped in the bond line.
The main aim of adhesive bonding is to produce durable adhesion of one part to another via a layer of adhesive. Ideally, the parts should not be separable from each other for the life of the assembly. However, good adhesion makes rework difficult. If the adhesion is too good, it may not be possible to disassemble the parts even with heating of the adhesive.
Hence, a need remains in the art for 1) a system or method for performing the vacuum bag process which reduces the amount of trapped air and 2) a system or method for providing adequate adhesion while ensuring that the assembly can be easily reworked.